The quality of loudspeaker audio has been increasing at a steady rate for over a century. In terms of timbre, there is a strong argument for saying recreation of a recorded sound is as good as it is going to get. However, the aspects of spatial quality have some way to go before an analogous plateau is reached. This discrepancy is due to the relatively recent arrival of multi-channel audio systems for home and vehicle use, providing the methods to reproduce sound in a way that seems engaging and aesthetically “natural.” Yet the vast majority of our musical recordings are stored with a two-channel stereo format that is recorded using two microphones.
There have been attempts at processing two-channel recordings so as to derive additional channels that contain reverberance information that can be played in an audio system including more than two loudspeakers. Such upmixing systems can be classified as spatial audio enhancers. Moreover, the goal of a commercial loudspeaker spatial audio system for music reproduction is to generally increase the enjoyment of the listening experience in a way that the listener can describe in terms of spatial aspects of the perceived sound. More generally, spatial audio enhancers take an audio recording, including one or more channels, and produce additional channels in order to enhance audio imagery. Examples, of previously developed spatial audio enhancers include the Dolby Pro Logic II™ system, the Maher “spatial enhancement” system, the Aarts/Irwan upmixer 2-to-5 channel upmixer, the Logic 7 2-to-7 upmixer and the Avendano/Jot upmixer.